The present invention relates generally to the field of nuclear magnetic resonance (NMR) tools for oil well logging and in particular to an active RF spoiler antenna for reducing the NMR signal from the bore hole in a side looking NMR tool.
NMR well logging instrument typically include a permanent magnet to induce a static magnetic field in the earth formations and a transmitting antenna, positioned near the magnet and shaped so that a pulse of radio frequency (RF) power conducted through the antenna induces an RF magnetic field in the earth formation. The RF magnetic field is generally orthogonal to the static magnetic field. After an RF pulse, voltages are induced in a receiving antenna by precessional rotation of nuclear spin axes of hydrogen or other nuclei about the static magnetic field. The precessional rotation occurs in an excitation region where the static magnetic field strength corresponds to the frequency of RF magnetic field. A sequence of RF pulses can be designed to manipulate the nuclear magnetization, so that different aspects of the NMR properties of the formation can be obtained. For NMR well logging the most common sequence is the CPMG sequence that comprises one excitation pulse and a plurality of refocusing pulses.
A xe2x80x9cside-lookingxe2x80x9d NMR tool is sensitive to NMR excitation on one side of the tool and less sensitive to NMR excitation on the other side. The more sensitive side of the tool is typically pressed against the side wall of a borehole adjacent a formation, thereby providing minimum separation between the NMR tool""s RF field generating assembly and the formation volume of NMR investigation. The less sensitive side of the tool is thus exposed to the bore hole. This operational NMR technique is most effective when the borehole diameter is much greater than the diameter of the NMR tool.
Typically, side-looking NMR tools set up static and RF magnetic field distributions in a particular relationship to achieve maximum NMR sensitivity on one side of the NMR tool. These conventional side looking NMR techniques are well known in the art, as taught in the following patents: U.S. Pat. No. 5,055,787, Kleinberg et al., entitled Borehole Measurements Of NMR Characteristics Of Earth Formation; U.S. Pat. No. 5,488,342, Hanley, entitled Magnet Assembly For NMR; U.S. pat. No. 5,646,528, Hanley, entitled Magnet Assembly; and WO 9942858, Prammer et al., entitled Eccentric NMR Well Logging Apparatus And Method.
The ""787 patent teaches a side-looking NMR tool which generates a static magnetic field which results in a sensitive volume on only the front side of the tool. The sensitive region in front of the ""787 tool generates a field having a substantially zero gradient, while the region behind the ""787 tool has a relatively large gradient field. Consequently, the volume of the sensitive NMR region in front of the tool is much larger and contributes more significantly to the composite NMR signal, than does the NMR region behind the tool. The ""787 patent technique, however, is only practical when the sensitive volume in front of the tool is very close to the tool and therefore limits the available depth of NMR investigation. The ""787 tool design also requires a substantially zero gradient in the sensitive volume. Such a zero gradient is not always desirable, however, in NMR well logging, as a number of associated NMR techniques depend upon having a finite, known gradient within the NMR sensitive volume.
The ""342 patent teaches a NMR tool technique which provides a homogeneous region localized in front of the tool. The ""342 tool design overcomes the disadvantageous requirement of the sensitive volume being undesirably close to the NMR tool. The ""342 tool, however, suffers because the sensitive volume is not elongated along the longitudinal axis of the NMR tool or bore hole axis, which causes unacceptable errors due to motional effects.
Another possibility would be to design a NMR tool that generates a static field so that the resonant region behind the tool is so far away that it never encroaches into any reasonably expected borehole diameter. This, however, would either require stronger magnets than are currently being used, or a lowering of the tool operating frequency. Stronger magnets are undesirable because they increase the cost, weight and size of the instrument. Moreover, the stronger magnets may attach to the well bore casing, making it difficult or impossible to pass the NMR tool through the casing to the borehole. Moreover, lowering the tool frequency is not desirable, because it lowers the signal-to-noise ratio for the NMR measurement.
A more effective way to reduce the signal from the region behind the tool is with the use of an RF shield. This is done to a great extent in U.S. Pat. No. 5,055,787, cited above, where the tool body effectively shields the antenna; and discussed in the patents U.S. Pat. No. 5,646,528 and WO99/42858. The passive RF shield is typically positioned as far as possible from the front region in order not to spoil NMR tool sensitivity in the desired region and as close as possible to the back region for maximum effectiveness. It can be seen therefore that the effectiveness of the passive shield will eventually be limited by the diameter of the tool. If we can not achieve sufficient attenuation with a shield inside the tool we will have to adopt one of the following undesirable options: use the large magnet to move the rear region further away; reduce the signal from the front region; or place a shield outside the tool. Thus, neither approach presents a practicable solution.
The present invention provides an active RF shield, or RF spoiler antenna which overcomes the limitations of the known side looking NMR tool designs described above. It is an object of the present invention to minimize NMR sensitivity behind the tool where the NMR signal from the bore hole tends to erroneously contribute to the received NMR signal. The spoiler antenna provides a substantial reduction in sensitivity of a side-looking NMR tool in the region in the bore hole without a reduction in sensitivity in the desired region of investigation and without the necessity of larger magnets, larger tool diameters, or external shields.
In accordance with the present invention, a side-looking NMR probe comprises a magnet for inducing a static magnetic field in the region of interest; a first antenna assembly for inducing a radio frequency (RF) magnetic field and receiving signals from the region of interest; and a second antenna assembly for compensating the RF magnetic field so that the resultant RF field forcefully mismatches the static magnetic field inside of the bore hole in order to reduce contributions from the bore hole to the sensed NMR signal. The second antenna is preferably active only during a transmit period of the first antenna. In another preferred embodiment the second antenna is active only during the excitation RF pulse and not active during refocusing pulses. In a preferred embodiment the magnet has a magnetic dipole moment perpendicular to a line which passes through the effective centers of the first antenna assembly dipole moment and the second antenna assembly dipole moment. The first and the second antennas preferably comprise a soft magnetic core.
In specific embodiments of the invention the NMR tool includes driving circuits which switch the second antenna in and out of the circuit as required. If the second antenna is switched in and out of the antenna circuit, it will change the inductance of the circuit, and hence the resonant frequency. In a specific embodiment the inductance of the first antenna is at a level where this change is small, and can be ignored. In an alternative embodiment a dummy inductor is employed to maintain the resonant frequency of the first antenna.